a) Field of the Invention
The present invention relates to a castable refractory composition for use in the production of cast refractory pieces.
The invention also relates to a process of preparing such a castable refractory composition and to the cast refractory pieces that can be produced therefrom.
b) Brief Description of the Prior Art
It is known, to use refractory compositions that are formulated to be castable for the manufacture of monolithic refractory pieces of complex shape.
In practice, the known castable refractory compositions distinguish from each other in the form in which there are delivered: plastic form, dry form or liquid form. The know castable refractory composition may also be distinguished from each other in the way they are used: ramming, gunning, casting or trowelling.
The plastic compositions are generally used as such. The dry compositions must be mixed and humidified before being used.
Because of their low cost, hydraulic cements are widely used for the preparation of such compositions. However, hydraulic cements are difficult to use, especially because of their content in water during mixing, which generally results in very dense products. Another difficulty lies in the very strict conditions of curing and firing that must be met to minimize shrinking and fissuring problems. A further difficulty lies in a reduction in mechanical resistance that is due to the dehydration that occurs when the cast piece is heated to its temperature of use normal to the very first time.
Chemical binders are much more interesting than hydraulic cements for the manufacturer of castable compositions. Several kinds of chemical binders can be used, such as oxysulphates, oxychlorides like the one sold under the tradename SOREL CIMENT, sodium silicates like the one sold under the tradename "WATER-GLASS" and, more particularly, phosphates.
Phosphate binders have numerous advantages as compared to the others. First of all, they can be used as a binder with almost any kind of a refractory aggregate. They also permit to obtain, at ambient or low temperature, a controllable hardening. With phospate binders, the loss of mechanical resistance during the first curing are negligible. Moreover, the refractory products obtained with such binders usually have a higher resistance to abrasion, crushing and chemical corrosion, over a large range of temperature.
It is well known however that phosphate binders cannot be prepared by direct reaction of phosphoric acid H.sub.3 PO.sub.4 with a metallic oxide such as MgO or Al.sub.2 O.sub.3, because the chemical reaction is too violent. However, there are two different approaches to solve this problem: either inhibiting the basic component or inhibiting the acidic component that are used for the reaction. Heating of the component after mixing reactivates the inhibited component and allows it to react with the other one to achieve the required hardening.
Contact between the basic component and phosphate acid can be restricted by physical encapsulation with an agent that is not soluble in water, such as a fatty acid. However, the addition of such an organic substance is not wanted during use of the product, because it is a source of smoke and it gives porosity in the resulting products.
The second solution mentioned herein above consists in inhibiting the acidic component of the phosphate binder. Phosphoric acid can be made less reactive in three different ways: esterification, polymerization or partial neutralization.
The esterification consists in neutralizing at least one of the acidic function of the phosphoric acid by condensation with an organic substance having a hydroxyl group, such as ethanol (see U.S. Pat. Nos. 3,950,177 and 4,981,821). Once again, this results in addition of an organic substance that is not wanted. Moreover, esterified phosphoric acid is not readily available on the market.
Polymerization of phosphoric acid to make it less reactive is also known (see U.S. Pat. No. 4,174,226), as is the use of sodium polyphosphate in place of phosphoric acid (see U.S. Pat. No. 3,839,057). In this case however, the presence of an alkaline metal causes another problem, as it may form an eutectic having a low melting point by reaction with the refractory oxide.
It is also known that phosphoric acid may be neutralized with aluminum derivatives (see U.S. Pat. Nos. 4,828,926 and 4,833,576), with magnesium and/or with other metals such as chromium (see U.S. Pat. No. 4,737,192). However, to the Applicant's knowledge, it has never been suggested to use a phosphoric acid that has been neutralized as is proposed herein for the manufacture of a castable refractory composition.